In recent years, FPD (flat panel display) such as a liquid crystal display (LCD) has been broadly used. FPD is produced by laminating optical films having various functions through a pressure-sensitive adhesive. In such applications, excellent reliabilities such as the properties of causing no foaming or separation under high temperature and high humidity conditions (foaming and separation resistances), as well as the adhesiveness and transparency, are required for the pressure-sensitive adhesive.
As the method for improving such characteristics, adjustment of the molecular weight of acrylic polymers (see, for example, JP-A-1-178567), copolymerization of modified monomers and the like have been known. In addition, for the purpose of improving adhesiveness to plastics, a method for improving foaming and separation resistances by adding an additive agent such as a tackifier, and the like have been known. However, these methods are not sufficient in terms of the characteristics of foaming and separation resistances under high temperature and high humidity environment.
Further, a pressure-sensitive adhesive composition and a pressure-sensitive adhesive tape, which are excellent in transparency and separation resistance for plastics and prepared by blending a specific oligomer with a specific acrylic polymer, have been known (see, for example, JP-A-2005-15524 and JP-A-2005-255877). However, these have a problem in that their adhesiveness for glass under a humidified condition is insufficient.
In addition, there has been known a pressure-sensitive adhesive composition having excellent transparency, foaming resistance and adhesive property, which contains a polymer containing a carboxyl group-containing monomer as a constituent component and a polymer containing an amino group-containing monomer as a constituent component (see, Japanese Patent No. 3516035). Further, there has been known a pressure-sensitive adhesive composition which is prepared by mixing a (meth)acrylic acid ester polymer (A) having a weight-average molecular weight of from 500,000 to 2,500,000 and a (meth)acrylic acid ester polymer (B) having a weight-average molecular weight of from 5,000 to 500,000 at a specific weight ratio, wherein either one of the polymers (A) and (B) has a nitrogen-containing functional group (see, JP-A-2001-89731). However, in order to improve adhesiveness with glass, it is necessary to contain a large amount of the polymer (B) in this pressure-sensitive adhesive composition, which poses a problem as the yellowing caused by amine.
On the other hand, a method in which a silane coupling agent is added to an acrylic pressure-sensitive adhesive has been known as a method for improving adhesiveness for glass (see, for example, JP-A-7-3221). However, since the preservation stability of the silane coupling agent is poor in such a system, hydrolysis as well as dimerization and trimerization of the silane coupling agent progress when preserved for a prolonged period of time, thus posing a problem of reducing adhesiveness for glass.
In addition, there has been known a pressure-sensitive adhesive composition for optical member use, which contains two carboxyl group-containing acrylic polymers having different weight-average molecular weights, a silane coupling agent and a cross-linking agent (see, JP-A-2004-263165). However, this composition has a problem in that it is insufficient in terms of the foaming and separation resistances with respect to plastics.